1. Technical Field
This invention relates to case-hardening technology, and more particularly to a method of monitoring a heat treatment with carbon penetration, carried out on workpieces placed in a furnace, of the type comprising the use of a probe for measuring the electrical resistance of a conductor placed in the presence of the atmosphere of the furnace, and the processing of the resistance values successively obtained by these measurements. The invention further relates to a probe for carrying out this method.
2. Description of the Related Art
The various developments which have taken place during the past few years in the art of gaseous cementation have pertained essentially to reducing the cycle times, to the quality and reproducibility of the treatments, as well as to the gas consumption and improvement of the safety conditions.
This research has led to supersaturating atmospheres, often out of equilibrium, maintained in agitated reactors, used at atmospheric pressure or under partial pressure of hydrocarbons in a vacuum furnace.
The values of the carbon potential carried out by indirect measurement, based on residual gases such as H.sub.2 O, CO.sub.2, or O.sub.2, can no longer be validly defined.
In treatment under vacuum or in a supersaturating atmosphere, where the release of the carbon does not take place via cracking of the CO molecule but by the direct decomposition of a hydrocarbon, the concept of equilibrium potential no longer exists and could be replaced by the term "kinetic potential," i.e., enrichment according to a linear law (in the usual range of cementation) as a function of time.
A method of monitoring and regulating completely adapted for following these reactions is the direct measurement of the carbon flux.
The principle consists in following in real time the carbon which enters at the surface of the measuring element, either by weighing (thermobalance or strip system) or by gas analysis after secondary reaction of the carbon with a gas in an system isolated from the reactor, or by measurement of the resistance of a detector as a function of its carbon enrichment.
The first principle, based on weighing an element, can hardly be used in an industrial reactor, for the electronic balance is very sensitive to jolts and vibrations. As for measurement on foil, the values are only intermittent and indicative of a state during a brief period of time.
The second method, devised by Meyer and Schmidt, consists in using a carbon-flux probe in which the carbon diffuses from the atmosphere of the furnace into a thin-walled steel tube; a decarburizing atmosphere based on humid N.sub.2 and H.sub.2 circulates within the tube. The carbon flux is determined from the CO/CO.sub.2 content of this atmosphere (see U.S. Pat. No. 3,843,419, for example).
The main drawback of this system is that the analysis does not take place in situ and the measurement chain based on infrared analyzers becomes complex and inaccurate.
The other system, developed by Joachin Wunning, uses the resistance method: a detector in the form of a very fine, short wire is enriched in the atmosphere of the furnace, and the measurement of its electrical resistance gives an indication of its carbon content, hence of the carbon flux as a function of time. At regular intervals, the probe is decarburized by an injection of H.sub.2, H.sub.2 O, and N.sub.2 around the detector (J. Wunning, Die C-Stromregelung bei der Gasaufkohlung, HTM 40, 1985).
This technique cannot be used in a vacuum reactor, for the introduction of a decarburizing gas shifts the gaseous reactions, weakens the enrichment kinetics, causes intergranular oxidation at the surface of the workpieces, and increases the working pressure.
The fragility of the wire and the complexity of its assembly after breakage (for soldering) must likewise be noted.
U.S. Pat. No. 2,935,866 to Schmidt and Wunning also describes a method of measurement using the electrical resistance of conductors. In this case, one detector is placed in an atmosphere having a known carbon content and another in the atmosphere to be measured.